1. Field of the Invention
This invention relates to an image transfer device for transferring electrophotographically formed and developed images to a transfer medium, and more particularly to such an image transfer device which can easily perform multicolor superposition transfers.
2. Description of the Prior Art
The known systems whereby developed images formed on a photosensitive medium may be transferred to a transfer medium include the system using a transfer roller to urge the transfer medium from the back thereof against the photosensitive medium, and the corona transfer system which utilizes corona discharge.
Color reproduction involves forming three or four different color images in superposed relationship with one another on a transfer medium and thus, requires image transfer to occur three or four times in order that a colored image may be transferred. During such image transfer, it is essential that no positional misregistration occur between the various color images transferred. For this purpose, there has been put into practice a system whereby a transfer medium may be securely wrapped about a transfer roller and used for a plurality of image transfers. See U.S. Pat. No. 3,612,677, for example.
In such transfer roller system, the contact of the transfer drum with the photosensitive medium has readily resulted in abrasion or pin-holes formed in the latter, which has in turn led to the likelihood of the life of the photosensitive medium being shortened.
Further, this system has suffered much from the risk of toner adhering to the surface of the transfer drum and when a different size of transfer paper is to be used, the back side of the transfer paper might readily become stained with the toner. To avoid this, a cleaner may effectively be provided to bear against the surface of the transfer drum to clean the same, whereas this would further require the provision of a cleaner control mechanism which would also lead to complication of the device. Moreover, if multicolor super-position transfer was effected with the transfer medium urged against the photosensitive medium by the transfer drum while an image transfer voltage being applied to the transfer medium, the image transfer field could not uniformly be imparted to portions of the transfer medium in which a developed image or images were already present, with a resultant tendency to aggravate the transfer efficiency of a subsequent image to be transferred. This has in turn led to an unsuccessful color balance between the superposed images transferred, which might also lead to a color image reproduction unfaithful to the original image. Furthermore, there has sometimes been the likelihood of giving rise to the hollowed phenomenon peculiar to the transfer roller, so called because the central part of a developed character image or the like failed to be transferred. The result might only be an incomplete color transfer. On the other hand, the corona image transfer is known to effect image transfer without the hollowed phenomenon taking place and has been put into practice for monochromatic reproduction, whereas no practical device capable of achieving good multicolor reproduction is known in the art to which the present invention pertains.